Femtosecond lasers are usually more complicated than other lasers emitting continuous-wave, Q-switched, or picosecond radiation. One reason for this is that femtosecond generation requires laser materials with a spectrally broad emission band, in comparison for example to the well-known laser material Nd:YAG, leaving a limited number of laser materials suitable for femtosecond generation. Additionally, femtosecond lasers need some group velocity dispersion compensation, which usually requires additional intra cavity elements, such as a prism pair, thereby adding complexity to the system. An example of a femtosecond laser is the green-pumped Ti:sapphire laser. More compactness is obtained by directly diode pumping suitable laser materials, such as Nd:glass, Cr:LiSAF, Yb:glass, etc (see for example in D. Kopf, et al., “Diode-pumped modelocked Nd:glass lasers using an A-FPSA”, Optics Letters, vol. 20, pp. 1169-1171, 1995; D. Kopf, et al., “Diode-pumped 100-fs passively modelocked Cr:LiSAF using an A-FPSA”, Optics Letters, vol. 19, pp. 2143-2145, 1994; C. Hönninger, et al., “Femtosecond Yb:YAG laser using semiconductor saturable absorbers”, Optics Letters, vol. 20, pp. 2402-2405, 1995). These laser systems, however, are not perfectly compact in the sense that they usually use two laser diodes as pump sources that are imaged into the laser crystal using imaging optics. The latter are relatively large in size and could still be made considerably more compact. Furthermore, the resonator comprises two arms that have to be aligned accurately with respect to each other and with respect to the pump beam, respectively, resulting in a number of high-accuracy adjustments to be performed.
A setup of this type is known from U.S. Pat. No. 5,987,049. This patent discloses a pulsed solid-state laser comprising a two-armed optical resonator with a solid-state laser medium and a semiconductor saturable absorber mirror device (SESAM) placed inside. A prism pair is incorporated for dispersion-compensating purposes. The achievable compactness of the setup is limited due to the positions of the SESAM and the prism pair at each end of the cavity arms. Quite commonly, focusing lenses with a focal length of 75 mm or longer are used to focus the pump light into the laser crystal through one of the curved cavity mirrors, following a delta-type laser cavity scheme. Such a cavity scheme essentially does not allow for straight-forward size reduction of the pump optics. Another approach (see for example S. Tsuda, et al., “Low-loss intracavity AlAs/AlGaAs saturable Bragg reflector for femtosecond mode locking in solid-state lasers”, Optics Letters, vol. 20, pp. 1406-1408, 1995) places the laser medium at the end of the laser cavity, thereby allowing for more compact pump focusing optics with a potentially shorter working distance and reducing the number of adjustments required. However, since one cavity end is taken by the laser medium, both the semiconductor element (semiconductor saturable absorber mirror, SESAM) and the prism sequence for dispersion compensation need to be placed toward the other end of the laser resonator. Since the spot size on the SESAM needs to be small enough for saturation in that setup, the focusing mirror towards that cavity end does not leave enough room for a prism pair to compensate for the group velocity dispersion. However a total of four prisms had to be implemented for that purpose.